Brushless DC motors have been used in many applications because of their inherent high inertia to weight ratios, their improved heatsinking capabilities, and the lack of brushes for commutation. In most applications, brushless motor drives, in addition to commutating the motor, are designed as transconductance amplifiers providing control of motor current in response to an input voltage command. Motor current determines motor torque. There are primarily two commutation schemes for brushless motors, commonly referred to as 6-step or squarewave commutation and sinusoidal or sinewave commutation.
In summary, squarewave commutation applies the amplifier voltage to the proper pair of motor terminals in discrete 60.degree. (electrical) increments based upon the motor rotor position and the motor back EMF (BEMF) and based upon the desired direction of rotation. This has two effects: 1) The variation in motor torque during a commutation interval is dependent upon the variation in motor torque constant, Kt, during that interval, and 2) The use of discrete 60.degree. switching points causes slight, but abrupt, changes in torque when the motor commutates from one interval to the next.
Sinusoidal commutation seeks to eliminate those changes in torque by applying the amplifier voltage in a continuous fashion to all three phases as a function of rotor position and motor BEMF. The disadvantages of sinusoidal commutation are increased rotor position sensor cost and increased electronic complexity of the amplifier.
Since the brushes are replaced with electronic switches and commutation circuitry, and since, in most cases, system efficiency is of prime importance, switching amplifiers have been preferred for driving brushless motors. A typical example is the use of Pulse Width Modulators (PWMs) at the current output stage of motor control systems and related switching topologies. The switching amplifiers, however, generate electromagnetic or radio frequency interference. There are situations where the electromagnetic interference, both conducted and radiated, generated by switching amplifiers is detrimental to system performance and alternative means of commutation and control must be used.
One method of reducing unwanted electromagnetic interference is the use of linear amplifiers to perform motor commutation and current control. There is no switching associated with the use of linear amplifiers, and therefore the electromagnetic frequency generated because of electric pulses can be completely eliminated.
Although linear amplifiers eliminate electromagnetic interference they also suffer from being less efficient than switching amplifiers; maximum power efficiency for a class AB amplifier is typically less than 75% . Efficiency becomes a critical design consideration in motor applications where high output power capability is required. Another design consideration is that as the power output requirements of amplifiers increase so does the need for multiple output power devices with thermal stabilizing circuits.
In order to increase current capability, many output power transistors may have to be connected in parallel. One of the problems facing a designer is the need for output power devices to equally share output current to prevent any one device from overheating. Overheating may cause thermal instability which ultimately leads to a phenomenon called thermal runaway where devices burn out due to temperature rise. Therefore for high current applications, designers have to implement thermal feedback amplifiers to improve thermal stability.
A further design consideration is to match the devices so that they can be connected in parallel. To gain component margin of safety, more devices are simply connected in parallel. Matching the output power devices, however, is another time consuming task. It requires an operator to manually or automatically test many transistors with a curve tracer and to locate transistors with exactly similar voltage-current curve characteristics. This procedure, however, does not guarantee predictable performance, because transistors' characteristics change differently over temperature and time.
Therefore, when space is at a premium and accurate individual device reliability is required, or when interaction between paralleled devices causes unpredictable performance, another method must be used. For a majority of applications switching amplifiers like Pulse Width Modulators were quite useful and sufficient. But for applications where electromagnetic interference is undesirable, a linear amplifier must be used which avoids the design problems discussed above.